About Me

No, not THAT Bob Hoover :-) (ie, Robert A. "Bob" Hoover from Tennessee and perhaps the best pilot in the history of flight.)
The problem is that all Roberts get Bob-ed at birth and there isn't much we can do about it. When posting something about aviation I generally use 'R.S.Hoover' to prevent confusion.

Thursday, November 23, 2006

VW - Compression Ratio

The use of rebuilt heads poses a problem for the unwary. Wanna not blow up your engine? Read on!

Doing the NumbersWe need to start with some definitions. The manuals differ on how they label things so for now, use the following:

Compression ratio is determined by dividing the total volume (V sub t) by the swept volume (V sub s).

Total volume is the sum of the chamber volume (V sub c), the deck volume (V sub d) and the swept volume (V sub s)

Chamber volume is not calculated, it is measured directly. (The Factory Workshop Manual offers a good explanation of the procedure.) For your basic overhaul you don’t need a very precise number but you must know the approximate chamber volume. An easy way to do this is to obtain a fat 50cc syringe from a veterinary or pharmacy, level the head with the valves and spark plug installed, then fill the chamber with water. Do it several times and average the results. Do it for all four chambers. When you calculate your compression ratio, use the smallest of the four chambers for chamber volume (V sub c).

If the chambers are larger than 50cc use a marble to take up some of the space. Determine the marble’s volume by dropping it into the half-filled syringe. It’s volume equals its displacement. (Eureka!) Use the same marble(s) in all four chambers and don’t forget to add its volume to the reading off the syringe.

Stroke is 69mm for a stock engine. If you’re rebuilding a 36hp, use 64mm. If your engine has a stroker crank, use that dimension. But don’t guess.

To calculate deck height, use the same formula but substitute deck height for stroke. Bore diameter is whatever pistons you are using. A stock 1600cc engine uses 85.5mm jugs.

Use millimeters for all of your calculations. 1mm is approximately equal to .0394 inches. Measure your deck height then convert it to millimeters by dividing the inch dimension by .0394

Since you’ll be using millimeters for your calculations, the answer will be in cubic millimeters. To convert to cubic centimeters, shove the decimal three places to the left.

For Example...

To illustrate, let me walk through the calculations using real data and real heads, a pair of rebuilts I have here in the shop.

The engine is a stock 1600cc dual-port. The jugs are 85.5mm in diameter, the crankshaft has a measured throw of 34.5 (i.e., a stroke of 69mm).

Since we use that same diameter when we calculate deck volume, let’s run the numbers and get it out of the way. 85.5 x 85.5 (that is, the diameter squared) equals 7,310.25. Now we multiply that by 0.785 (that is, the pi factor) to get 5,738.546. We will use that value to compute both swept volume and deck volume.

5,738.546 times the 69mm stroke gives us 395,959.6913 as our swept volume.

The deck height measured .065" Dividing that by .0394" gives us 1.649mm.

5,738.546 times the 1.649mm deck height gives us 9,462.86.

The combustion chamber measured 43.5 cubic centimeters so lets convert the other factors to cubic centimeters before we add them together.

(Note: Engine displacement is based on swept volume rather than total volume. Four times 396cc equals 1584, the displacement of a ‘1600' engine.)

Dividing the total volume by the sum of the chamber and deck volume gives us 449.0 divided by 53.0 or 8.346... which is our compression ratio. And that is too high.

A Bit of Background

This particular engine is in the shop because it blew a piston . . . hole about as big as a quarter, right through the top of the thing. And it was just overhauled, too.

How could such a thing happen? Easy. Kid talks his mom into buying him a VW. It’s a total POS but he’s convinced he can fix it up. Following the advice of the local guru, first thing the kid does is buy a pair of rebuilt heads and slap them on without bothering to check anything. But one of the heads had been flycut. Flycutting reduces the chamber volume, which raises the compression ratio. A combination of hot weather, a heavy foot, low octane gas and a high compression ratio lead to detonation. Naturally, the kid kept on driving. And of course he had to really keep his foot in it to get over them hills with only three cylinders.

The second jug blew a few minutes after the first, which convinced him to turn around and head for home. Amazingly, he managed to make it home on the two surviving cylinders. Of course, it cost him an engine. He now drives his mom’s Toyota and rails against veedubs as ‘nothing but junk.’ His mom, no dummy, won’t even let him check the air in the tires :-)

So you run the numbers and they say the compression ratio will be too high. What do we do now?

We put spacers under the cylinders, that’s what. Of course, we need to know how thick a spacer we should use but that turns out to be pretty simple, we merely turn the equation around.

More Numbers

With a total volume of 449cc, what volume chamber-plus-deck will give us 7.3:1?

To find out, just divide 449 by 7.3. You should get about sixty-one and half... 61.5cc’s.

And since you can’t change the chamber volume, lets get it out of the equation by subtracting it from the 61.5. 61.5 minus 43.5 equals 19.0cc, or about twice our original deck volume.

Since we measured the deck height as .065" the quick and dirty solution would be to double it by slapping a sixty-thou shim under the jugs and drive on. And it would work just fine, too. But quick & easy answers are often a bit too quick & easy. Remember, deck volume appears in both sides of the equation. If we increase our fixed volume . . . the volume of the deck plus the volume of the chamber . . . we have also increased our total volume. So before we dash off in all directions, lets run the numbers again. First, we’ll add 9.5cc to our total volume, making it 458.5 cc’s. Now we add 9.5cc’s to our fixed volume, making it 62.5cc. Now divide 458.5 by 62.5 and see what you get.

7.336?

Ah ha, said the mechanic. Ah ha indeed! Close enough.

(But what if the numbers had been off? In that case simply do the calculations over again, increasing deck height by ten thou (.25mm) each time until your compression ratio drops below 7.5 to 1.)

Okay, in this case the numbers worked out close enough. And a sixty-thou spacer is a standard item, if you call around. But be careful. Everyone carries tens, twenties and forties. They’ll tell you to stack them up to make a sixty but don’t do it, you want as few surfaces as possible. The good shops carry shims all the way up to ninty thou (.090") in increments of ten thou (.010") and only charge about eight bucks a set. cheep.

(Before posting this article I called around to verify availability and price of cylinder shims. A couple of outfits only carried the thinner three sizes, insisted it was okay to simply ‘stack ‘em up’ to get whatever thickness was required. The better shops carried the full range. Johnny’s Speed & Chrome sez stackemup, Mark Stephens’ shop was the other sort . . . “We got stock, ten thru ninety. Or we can polish you up a set.” I like their attitude.)

The range of available spacers brings up another point. The use of cylinder shims or spacers is the normal procedure used to adjust the compression ratio of a rebuilt engine and all of the better after-market suppliers keep the standard sizes on the shelf, ready for immediate delivery. But if you want a size that isn’t in stock, the price goes up dramatically since someone will have to chuck a set of spacers in a surface grinder and bring them down to the size you need. This can cost up to forty dollars.

So let’s say your numbers tell you to use a spacer exactly .035" thick. Unless you’re out to win a race, don’t do it. Order a set of forties and drive on. But don’t stack up your shims. It increases the risk of leaks.

The rule here is to opt for the next larger size of standard shims that will give you the compression ratio you need. And it’s handy to know that the paper gasket in the standard overhaul kit is about .008" thick. I don’t use gaskets on my engines since I prefer a metal-to-metal joint between the cylinders and crankcase, but in a special case with stock jugs, I might use them.

Sermonette

Stock Volkswagen engines (for gasoline) have used compression ratios as low as 5.8:1 and as high as 8.5:1. Given today’s gasoline, the wiser course is to err on the low side, with a compression ratio of no more than 7.5:1, and a whole lot less if you buy your gas at a Pemex station. (Air cooled engines for the domestic Mexican market are fitted with dished pistons that give a CR of 6.6:1, reflecting the low octane rating of Pemex regular gasoline.)

Rebuilt parts are liable to vary wildly from the dimension of stock, original Volkswagen equipment. When someone sez a part is ‘stock’ it don’t mean a thing until you clap a caliper on it and see for yourself that it falls within stock specs.

In an earlier series of posts I wrote about filter/pump adaptors that were machined about .006" undersize, rendering them useless. On returning the part to the store I demonstrated that it could not work as intended. Later I visited the same store and saw the same filter-pump in the showcase, waiting for the next sucker to come along.

When it comes to parts, rebuilt or new, it’s insane to trust the veracity of the guy behind the parts counter. A few simple tests and measurements not only eliminates confusion it usually results in a better engine.